Circadian regulation of Ca 1.2 expression by RORα in the mouse heart.

Background: In addition to show autonomous beating rhythmicity, the physiological functions of the heart present daily periodic oscillations. Notably the ventricular repolarization itself varies throughout the circadian cycle which was mainly related to the periodic expression of K channels. However, the involvement of the L-type Ca channel (Ca 1.2 encoded by gene) in these circadian variations remains elusive.

Methods: We used a transgenic mouse model (PCa-luc) that expresses the luciferase reporter under the control of the cardiac promoter and analyzed promoter activity by bioluminescent imaging, qPCR, immunoblot, Chromatin immunoprecipitation assay (ChIP) and Ca 1.2 activity.

Results: Under normal 12:12h light-dark cycle, we observed a biphasic diurnal variation of promoter activities peaking at 9 and 19.5 Zeitgeber time (ZT). This was associated with a periodicity of mRNA levels preceding 24-h oscillations of Ca 1.2 protein levels in ventricle (with a 1.5 h phase shift) but not in atrial heart tissues. The periodicity of promoter activities and Ca 1.2 proteins, which correlated with biphasic oscillations of L-type Ca current conductance, persisted in isolated ventricular cardiomyocytes from PCa-Luc mice over the course of the 24-h cycle, suggesting an endogenous cardiac circadian regulation. Comparison of 24-h temporal patterns of clock gene expressions in ventricles and atrial tissues of the same mice revealed conserved circadian oscillations of the core clock genes except for the retinoid-related orphan receptor α gene (RORα), which remained constant throughout the course of a day in atrial tissues. we found that RORα is recruited to two specific regions on the promoter and that incubation with specific RORα inhibitor disrupted 24-h oscillations of ventricular promoter activities and Ca 1.2 protein levels. Similar results were observed for pore forming subunits of the K transient outward currents, K 4.2 and K 4.3.

Conclusions: These findings raise the possibility that the RORα-dependent rhythmic regulation of cardiac Ca 1.2 and K 4.2/4.3 throughout the daily cycle may play an important role in physiopathology of heart function.